In the simultaneously filed application of Apostolos, Boland, and Stromswold for an Acquisition System Employing Circular Array, there is disclosed a method and apparatus for operating on the outputs of a circular array of antenna elements so that a device for generating a two-dimensional Fourier transform in space and time, such as a two-dimensional compressive receiver, indicates the directions and frequencies of many signals simultaneously when fed the resultant signals. The Apostolos et al. application is hereby incorporated by reference.
Reference may be had to that application for details of the system. Briefly, however, the outputs of the circular array of that system are fed to a device for generating a spatial Fourier transform. The resultant outputs representing each spatial-frequency component are fed to a separate correction circuit associated with that spatial frequency. Each correction circuit is associated with a different integer, and its transfer function is the frequency-independent factor of the antenna pattern generated by driving the elements of the linear array with signals whose relative phases advance, in one circuit around the array, through a number of complete cycles that is equal to the integer with which the correction circuit is associated.
As is pointed out in the Apostolos et al. specification, the correction networks can be either analog or digital in form, the choice typically depending on whether the initial Fourier transformation is performed in an analog manner or digitally. For relatively high frequencies, the correction circuits may be difficult to implement digitally, because it is necessary to perform a convolution of the spatial-transform outputs with the impulse response corresponding to the correction-circuit transfer function. Also, since the number of correction networks is equal to the number of array elements, the hardware cost associated with implementation of the correction networks can be significant if each individual correction network requires complicated circuitry.
It is among the objects of the present invention to permit the correction networks to be implemented, when the initial transformation is performed digitally, without convolution and, when the initial transformation is performed in an analog manner, without frequency-dependent circuitry.